The present invention relates generally to coatings for ceramics, and, more particularly, to apparatus and methods to protect ceramic surfaces in high temperature, moisture rich environments.
Both airfoils and combustors made from silicon nitride or silicon carbide have the potential to appreciably increase the operating temperatures of turbine engines. The high temperature and high pressure environment of the turbine engine as well as the high gas velocity can cause erosion of silicon based ceramics. The mechanism of some of the erosion loss is due to the formation of SiO2. Typically, combustion gas environments, including turbine engines, contain about 10% water vapor. Oxygen containing water in the turbine reacts with silicon nitride and silicon carbide to form silica scale on silicon based ceramic surfaces. Water vapor can also react with the silica scale to form silicon hydroxide, which is volatile. Evaporation of silicon hydroxide from ceramic surfaces and erosion of ceramic caused by high speed combustion gases passing over ceramic surfaces leads to the loss of ceramic material from ceramic combustor and turbine components at rates of a few microns per hour.
U.S. Pat. No. 6,159,553 and US 202136835 A1 show one method of protecting ceramic coatings. Tantalum oxide alloyed with lanthanum oxide provides an environmental coating (EBC). However, tantalum oxide permits diffusion of oxygen, resulting in the formation of a SiO2 layer below the tantalum oxide layer. As long as the SiO2 layer is thin, <10 microns, and free of stress cracking, it provides a barrier to additional oxidation. Protection for over 500 hours at 2200° F. (1200° C.) was observed in a Keiser rig at approximately 1.5 atmospheres of water vapor. Protection in a cyclic oxidation air environment was demonstrated at 2400° F. (1315° C.) for 2000 hours after which the interfacial silica layer was less than 10 microns and un-cracked.
However, thicker silica layers in the range of 20 to 25 microns developed during 500 hours in a Keiser test rig at 0.3 to 2 atmospheres of water vapor and 2400° F. (1315° C.). The thicker SiO2 scale was predominately crystobalite, and was excessively cracked during cooling from the operating temperature. In addition, to compromising the adhesion of the tantalum oxide coating, the crystobalite reduced the mechanical strength of the Si3N4 substrate. As disclosed in U.S. Pat. No. 6,159,553, line of sight coating techniques such as plasma spray have been used to apply coatings such as tantalum oxide. However, plasma spray does not deposit a uniform thickness coating onto complex shapes such as multi-airfoil components.
Published US patent application 2002/0098391 by Tanaka et al discloses the use of rare earth silicates to form a protective coating to a silicon based substrate ceramic material. But the process disclosed by Tanaka limits the coating composition because it allows interaction of the coating with the substrate. Specifically, in the Tanaka reference, concentrations of rare earth oxides, in excess of the amounts specifically called for, will be free to diffuse from the substrate into the coatings causing undesirable changes (i.e., weakened oxidation sites) in the coating material. Thus, the manufacturing processes required to achieve the correct compositions required in Tanaka are more critical and therefore more expensive.
The Tanaka reference additionally requires a relatively thick coat of silicate or disilicate in the range of 50 to 500 microns. Thick dense coatings are well known to reduce the strength of the substrate. In addition, thick coatings can not beneficially be applied to airfoil like applications as they add too much weight to the airfoil and, because space is limited, result in the thickness of the airfoil having to be reduced and hence its load carrying capacity. In contrast, thick coatings are viable on lower-stress combustor and turbine shroud components. The sintering process for the coating disclosed by the Tanaka reference also requires a relatively high sintering temperature in the range 1650 to 1800° C. (3000 to 3270° F.). This high sintering temperature can limit and compromise the strength properties of the substrate. In addition, the functional cycle times and operating temperatures reported in Tanaka are inadequate to meet turbine engine requirements. For instance, Tanaka reports coated parts surviving 1400° C. (2550° F.) for 30 cycles (1 cycle is starting at room temperature, raising to operating temperature and then returning), whereas a turbine engine might require components to survive 1400° C. (2550° F.) for 1000 cycles. Further, the testing conducted in the Tanaka reference was reported in air which is less demanding than the moist environment of a turbine engine. Finally, the Tanaka reference discloses an approach that requires sintering additives for the silicon nitride substrate that must be of the same rare earth used in the coating. This requirement would eliminate possible unfavorable reactions between the coating and the substrate's sintering aid, but limits the range of coatings that can be used. Thus for example, if yttrium compounds are used as sintering aids in the formation of the substrate, then the oxidation coating of Tanaka must also be of yttrium compounds to avoid unfavorable reactions. For example a lanthanum compound is beneficial as a sintering aid in the silicon nitride, but is detrimental in the protective coatings. Tanaka's approach avoids lanthanum compound additions in the silicon nitride to avoid their diffusion into the coatings.
As can be seen, there is a need for an improved coating and method to apply the coating for a high temperature (>2200° F. (1200° C.)) barrier between an environmental coating and a substrate of silicon nitride or silicon carbide. There is also a need for a diffusion coating that will prevent migration of cations out of a silicon-based substrate. There is as well a need to coat complex parts with a uniform dense oxidation coating at a minimal cost.